With reference to FIG. 1, gas turbines 1 have a compressor 2, a combustion device 3 and a turbine 4; the compressor 2 generates high pressure air that is supplied together with a fuel to the combustion device 3 where they are combusted to generate hot gases that are expanded in the turbine 4 (high pressure turbine); from the turbine 4 exhaust gases are discharged.
Sequential combustion gas turbines have a second combustion device 5 downstream of the high pressure turbine 4 where additional fuel is supplied into the exhaust gases and it is combusted to generate hot gases that are then expanded in a second turbine 6 (low pressure turbine).
Within the second combustion device 5, together with fuel, also air is injected; this air has mainly the functions of helping fuel penetration into the exhaust gases flowing through the second combustion device, and preventing an early fuel combustion immediately after injection.
Since the exhaust gases that pass through the second combustion device 5 have a large pressure (because only partial expansion occurs in the high pressure turbine 4), the air that is injected into the second combustion device 5 is drawn from the last stage of the compressor 2.
This cooling scheme has some drawbacks.
In fact, since at the last stage of the compressor 2 the air has a large pressure, drawing air (and thus preventing this air from combustion and expansion) has a large negative impact on the performances and efficiency of the gas turbine.